Absolute range fuze system using limiting or agc

ABSTRACT

1. An FM doppler fuze system comprising means for transmitting a signal having a carrier frequency modulated by a band of random noise, means for mixing the transmitted signal with a return echo signal modified by the doppler effect of the relative movement between the fuze and a target, low pass filter means for deriving an output adapted to actuate the fuze from the low frequency doppler portion of the output from said mixing means, and means coupled between said mixing means and said low pass filter to maintain said output proportional to the distance from said fuze to said target and independent of the size of said target.

United States Patent Adrian March 6, 1973 ABSOLUTE RANGE FUZE SYSTEM USING LIMITING OR AGC Donald J. Adrian, Arlington, Calif.

The United States Government represented by the Secretary of the Navy Filed: March 11, 1957 App]. No.: 645,385

Inventor:

[73] Assignee:

[56] References Cited UNITED STATES PATENTS 8/1941 Guanella ..343/14 FOREIGN PATENTS OR APPLICATIONS 2/1955 Great Britain Primary Examiner-Benjamin A. Borchelt Assistant Examiner-G. E. Montone AttorneyW. O. Quesenberry and J. M. St. Amand [5 7] ABSTRACT I. An FM doppler fuze system comprising means for transmitting a signal having a carrier frequency m0du lated by a band of random noise, means for mixing the transmitted signal with a return echo signal modified by the doppler effect of the relative movement between the fuze and a target, low pass filter means for deriving an output adapted to actuate the fuze from the low frequency doppler portion of the output from said mixing means, and means coupled between said mixing means and said low pass filter to maintain said output proportional to the distance from said fuze to said target and independent of the size of said target.

3 Claims, 3 Drawing Figures ,1! I I2 3 1 I I NOISE BAND-PASS F M. MO D pDLATQR SOURCE F'LTER TRANSMITTER A.G.C. OUTPUT LOW PASS MIXER 0R F'LTER LIMITER PATENTEBHAR 6 E373 SHEET 2 BF 2 l4 7 f" l3 NQ|3E BAND PASS F.M. MgggLATOR SOURCE FILTER TRANSMITTER [I8 IE7 OUTPUT LOW-PASS A.G.C. OR

FILTER LIMITER MIXER WW D\/B7T DONALD J. ADRIAN I NVENTOR.

ATTORNEYS ABSOLUTE RANGE FUZE SYSTEM USING LIMITING R AGC The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to a fuze system and more particularly to a random frequency modulation correlation fuze system giving absolute range indication.

Sinusoidal and other types of periodic modulation have been previously used in frequency modulated doppler fuze systems to give some range attenuation but the cutoff in these systems has been very poor and ambiguous due to the periodic modulation.

A random frequency modulation correlation fuze system is disclosed in the copending application of Thomas B. Whiteley and Donald J. Adrian, Ser. No. 566,318, filed Feb. 17, 1956. This system utilizes band limited random noise for frequency modulating a carrier to provide rapid, unambiguous range cutoff in a doppler signal obtained by mixing the transmitted signal with the echo return signal from the target and filtering and rectifying the output. This system, however, operates at a larger distance from a large target than it does from a small target and therefore does not provide an absolute range indication when used against all types of targets.

In the fuze system of the present invention, a white noise source is utilized to provide a substantially uniform frequency band of noise at random frequencies which is passed through a band pass filter and used to frequency modulate the carrier frequency which is transmitted in the direction of the target. The echo return signal received from the target is passed through an automatic gain control'or a limiter circuit, and is then mixed with a portion of the transmitted signal, the return signal being phase modulated by the doppler effect of the movement between the missile and the target. The output of the cosine law mixer provides the auto-correlation function of the power spectrum which is then passed through a low-pass filter which transmits the doppler frequency output, and the doppler output is then used to trigger the fuze with a suitable delay provided if desired. The amplitude of the output signal will be dependent on range alone and will not be affected by the size of the target.

One object of the present invention is to provide a fuze system having a rapid and unambiguous range cutoff which will provide a firing signal which is independent of target reflection properties and thus give an absolute range indication.

The object of the present invention is to provide a doppler fuze system which can be set to operate at a specific range from all targets.

A still further object of the present invention is to provide a random frequency modulation correlation fuze system wherein the fuze will not operate at a larger distance from a large target than from a small one.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a block diagram of a preferred embodiment of the invention.

FIG. 2 is a modification of the embodiment shown in FIG. 1.

FIG. 3 is a diagram illustrating the useful fuze signal which is the doppler frequency having an amplitude proportional to a Gaussian function of the range. I

Referring now to the drawing in detail, one preferred embodiment of the present invention is illustrated in the block diagram of FIG. 1 wherein a white noise source 11 provides a continuous uniform spectrum of random frequency noise which is passed through a band pass filter 12. The band of white noise which is passed through the filter 12 is utilized to modulate the carrier frequency in the frequency modulator and transmitter 13 which transmits the noise modulated frequency modulation signal in the direction of the target through the antenna 14.

The fuze of the present invention is assumed to be mounted in a moving missile which is guided in a path to approach the target with the radio frequency energy from the fuze being transmitted from the antenna 14 toward the target and a return echo signal being received therefrom through the antenna 16.

It will be apparent that different targets of different shapes and sizes will have different reflecting properties for the radio frequency signal. The reflecting properties will also vary with the aspect of approach of the missile towards the target. Since a fuze is generally triggered when the amplitude of the return signal reaches a predetermined value, it will be apparent that the fuze will be triggered at a larger distance from a large target or from a target which presents a larger reflecting surface at the particular aspect or angle of approach of the missile towards the target than it will from a smaller target or one presenting a smaller reflecting surface at the particular aspect of approach. It is obvious that this variable response is undesirable in most cases, and although this can be partially compensated by presetting the time delay, the system of the present invention, by eliminating the variable factor introduced by different reflecting properties of difi'erent targets and different angles of approach, will simplify the time delay problem in connection with missile fuzes.

The echo signal returned from the target aircraft is modified or phase modulated by the doppler effect of the relative movement between the missile and the target aircraft and is received through the antenna 16.

In the prior system for a random frequency modulation correlation fuze, as disclosed in the copending application noted supra, the return signal is applied directly to the mixer but at any specific range, particularly at certain critical ranges where the fuze is adapted to fire, the amplitude of the return signal will vary depending on the size, shape and other characteristics of the target, as well as the aspect of approach of the missile and therefore, since the fuze is adapted to fire at a certain amplitude of signal, the fuze may fire at different distances from the target. The signal in the embodiment of FIG. 1 is preferably passed through an automatic gain control or limiter circuit 17 before it is applied to the cosine law mixer 18, where it is mixed with a portion of the signal from the transmitter 13.

Without the addition of automatic gain control or limiting in the receiving channel, the fuze signal is of the form f(T) cos 0. -r e wheref(T) is a function of the target reflection properties, I) is the carrier frequency and 1' is the time for the transmitted signal to travel to the target and back. By putting a limiter in the receiving channel,'function T is limited to a value K and the fuze signal is a range function independent of the reflection properties of the target.

In a microwave system the limiter circuit 17 should preferably be placed between the mixer 18 and the low pass filer 19, as shown in FIG. 2 since it is difficult at present to design a suitable microwave limiter. The functioning of the system will be the same and the return signal as well as the output from the mixer will be limited in either case. However the system will not function to accomplish the objects of the present invention if the limiter is placed after the low pass filer 19, since the low pass filter output of a highly reflective target will have a larger amplitude at longer ranges and will fire early.

The action of the limiter circuit either between the mixer 18 and filter 19 or before the mixer 18 is to limit the amplitude of the entire signal at all ranges, and regardless of the reflective properties of the target the output from the low pass filer 19 will be the same and similar to the output illustrated in FIG. 3 of with some squaring of the peaks for larger return signals. Limiting could also be placed between the transmitter and mixer, if desired, to reduce the effects of stray amplitude modulation.

The output of the cosine law mixer 18 is passed through a low pass filer 19 which is set to pass all the expected doppler frequencies, and the output from the low pass filter 19 will provide a trigger signal when it reaches a suitable level for firing the detonation circuits of the fuze (not shown).

The automatic gain control, or limiter circuit 17, may be of conventional construction for the particular frequency ranges involved and will normally be adjusted to provide the optimum results for use of the system on any particular fuze or in any particular tactical situation which is anticipated.

In addition to its use as a fuze the Range Fuze System of the present invention could be used as a miss distance indicater. The detector envelope could be telemetered to a ground station or other recording station. The amplitude of the envelope could be calibrated in terms of miss distance between missile and target.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefor to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. An FM doppler fuze system comprising means for transmitting a signal having a carrier frequency modulated by a band of random noise, means for mixing the transmitted signal with a return echo signal modified by the doppler effect of the relative movement between the fuze and a target, low pass filter means for deriving an output adapted to actuate the fuze from the low frequency doppler portion of the output from said mixing means, and means coupled between said mixing means and said low pass filter to maintain said output proportional to the distance from said fuze to said target and independent of the size of said target.

2. The fuze system comprising a white noise source adapted to produce a uniform spectrum of random frequency signals, a band pass filter operatively associated with said white noise source and adapted to ass a band of said noise, means for modulating a carrier frequency with said band of noise and transmitting a signal in the direction of the target, means adapted to mix a portion of said transmitted signal and a return echo signal from a target, low pass filter means adapted to pass the low frequency band from the output of said mixer for actuating a fuze, and means coupled between said mixing means and said low pass filter to maintain said output proportional to the distance from said fuze to said target and independent of the size of said target.

3. A fuze system comprising a white source adapted to produce a uniform spectrum of random frequency signals, a band pass filter operatively associated with said white noise source and adapted to pass a sharply defined band of said noise, means including an FM modulator and transmitter for modulating a carrier frequency with said band of noise and transmitting a signal in the direction of the target, means including a cosine law mixer adapted to receive a portion of said transmitter signal and a return echo signal from said target, means including a low pass filter adapted to pass the band of expected doppler frequencies from the output of said mixer for actuating a fuze, and means coupled between said mixing means and said low pass filter to maintain said output proportional to the distance from said fuze to said target and independent of the size of said target. 

1. An FM doppler fuze system comprising means for transmitting a signal having a carrier frequency modulated by a band of random noise, means for mixing the transmitted signal with a return echo signal modified by the doppler effect of the relative movement between the fuze and a target, low pass filter means for deriving an output adapted to actuate the fuze from the low frequency doppler portion of the output from said mixing means, and means coupled between said mixing means and said low pass filter to maintain said output proportional to the distance from said fuze to said target and independent of the size of said target.
 1. An FM doppler fuze system comprising means for transmitting a signal having a carrier frequency modulated by a band of random noise, means for mixing the transmitted signal with a return echo signal modified by the doppler effect of the relative movement between the fuze and a target, low pass filter means for deriving an output adapted to actuate the fuze from the low frequency doppler portion of the output from said mixing means, and means coupled between said mixing means and said low pass filter to maintain said output proportional to the distance from said fuze to said target and independent of the size of said target.
 2. The fuze system comprising a white noise source adapted to produce a uniform spectrum of random frequency signals, a band pass filter operatively associated with said white noise source and adapted to pass a band of said noise, means for modulating a carrier frequency with said band of noise and transmitting a signal in the direction of the target, means adapted to mix a portion of said transmitted signal and a return echo signal from a target, low pass filter means adapted to pass the low frequency band from the output of said mixer for actuating a fuze, and means coupled between said mixing means and said low pass filter to maintain said output proportional to the distance from said fuze to said target and independent of the size of said target. 